A locomotive and its bogies are subject to phenomena, called nose lift, which tend to make them pivot about their transverse axis, causing variations in the load on the axles.
The nose lift of the chassis is related to the fact that the resistant force applied to the chassis by the string of cars is located higher than the traction force transmitted by the bogie to the chassis.
The nose lift of the bogie is related to the fact that the resistant force applied to the bogie by the chassis is located higher than the traction force at the level of the rails. It is also related, in the bogies comprising motors suspended by the nose, to the fact that the motors are supported on the bogie chassis, causing unburdening of one axle and overloading of the other axle.
This nose lift movement causes certain axles to be unloaded while other axles are overloaded. There is a lessening of total traction force and an appearance of skating.
To prevent nose lift movement, solutions have been found which relate to the connections between the chassis and the bogies. One solution consists of locating the connecting points between the bogie and the chassis as close as possible to the level of the rails. This solution is called "low traction." A construction of this type consists of rods connecting the chassis to the bogies, the points of attachment to the bogies being located as low as possible.
It has been conceived to modulate the supply of the electric motors as a function of instantaneous variations in the rotational speed of the axles. The only function of this system is to prevent skating of the least loaded axles.